Most oil and gas wells eventually require some form of stimulation to enhance hydrocarbon flow and make or keep them economically viable. The servicing of oil and gas wells to stimulate production requires the pumping of fluids under high pressure. The fluids are generally corrosive and abrasive because they are frequently laden with corrosive acids and abrasive proppants such as sharp sand.
In some wells, stimulation to improve production can be accomplished at moderate pressure which may be safely contained by blowout preventers (BOPs) and, therefore, stimulation fluids may be pumped directly through a valve attached to the BOPs. This procedure is adopted to minimize expense and to permit full access to the well casing with downhole tools during the well servicing operation. It has been demonstrated that it is advantageous to have full access, or substantially full access, to a well casing during a well stimulation treatment. Full access to the casing permits use of downhole tools which are often required, or at least advantageously used during a stimulation treatment.
An apparatus for providing full access to the casing while permitting stimulation treatments at extreme pressures that approach a burst pressure rating of the casing is described in Applicant's U.S. Pat. No. 5,819,851 which issued on Oct. 13, 1998 and is entitled BLOWOUT PREVENTER PROTECTOR FOR USE DURING HIGH PRESSURE OIL/GAS WELL STIMULATION. The patent describes an apparatus for protecting BOPs during well treatments to stimulate production. The apparatus includes a hollow spool that has spaced apart inner and outer side walls that define an annular cavity. A mandrel is forcibly reciprocatable in the cavity. The mandrel includes an annular seal at the bottom end for sealingly engaging a bit guide attached to the top end of the casing. The apparatus is mounted above a BOP attached to a casing spool of the well before well stimulation procedures have begun. The mandrel is stroked down through the BOP to protect it from exposure to fluid pressure as well as to abrasive and corrosive well stimulation fluids, especially extreme pressure and abrasive proppants. The BOP protector provides a simple, easy to operate apparatus for protecting BOPs which provides full access to the well casing with well servicing tools to facilitate well stimulation at pressures approaching the burst pressure rating of the well casing.
The BOP protector has been readily accepted by the industry and has been proven to be an effective tool which reduces the cost of well stimulation treatments while enabling an ultimate choice of treatment options. However, further improvements are still desirable because the BOP protector described in U.S. Pat. No. 5,819,851 is a hydraulic unit which is mounted above the BOPs during an entire stimulation process. This raises the high pressure valve which controls the flow of stimulation fluids well above a top of the BOPs, which complicates access and reduces the run-in room for perforating gun strings, and other lengthy tools. Consequently, a low profile BOP protector would be advantageous to lower the position of the high pressure valve for easy access during stimulation processes. In addition, a mechanical lockdown mechanism for securing the BOP protector mandrel in an operative position is considered more reliable because a source of pressurized hydraulic fluid is not required.
An apparatus and method of isolating a well tree located on an oil or gas well from the effects of high pressure or corrosion caused by stimulation of a well is described in Applicant's U.S. Pat. No. 4,867,243 which issued on Sep. 19, 1989 and is entitled WELLHEAD ISOLATION TOOL AND SETTING TOOL AND METHOD OF USING SAME. The patent describes an apparatus to permit the injection of fluids, gases, solid particles or mixtures thereof through a well tree while protecting the well tree during well stimulation treatments. The apparatus includes a single hydraulic cylinder supported in an axial alignment over a well tree by at least two elongated support rods. The hydraulic cylinder support rods are connected between a base plate and a hydraulic cylinder support plate for supporting the hydraulic cylinder above the well tree at a distance approximately equal to the height of the production tree. The apparatus permits the insertion of a single length of high pressure tubing through any well tree regardless of its height. Once the high pressure tubing is seated in a well tubing or casing, the hydraulic cylinder, hydraulic cylinder plate and support rods are removed to provide 360.degree. access to a high pressure valve attached to the top of the high pressure tubing. The bottom end of the high pressure tubing has a packoff nipple assembly which is inserted into the production tubing or casing and seals against the inner wall. The extent to which the high pressure tubing extends into the production tubing or casing is unimportant so long as the packoff nipple assembly is sealed against the inner wall. Consequently, variations in the length of the production tree are of no consequence and a lockdown mechanism with a short reach is adequate. Consequently, there exists a need for a mechanical lockdown mechanism that provides a broad range of adjustment to permit packoff with a fixed packoff surface in a wellhead.